1. Field of the Invention
This invention relates to a method for monitoring chemical processes, and more specifically the invention relates to a high-throughput method for micro-sampling and analyzing process fluids.
2. Background of the Invention
Processing used nuclear fuel to separate fission products from valuable fissile and fertile uranium and transuranic elements can be done in a multitude of ways. One way is through electrochemical processing. In electrochemical processing, used uranic nuclear fuel contacts the anode of an electrochemical cell, the electrolyte of which comprises molten salts (often a LiCl—KCl eutectic). When voltage is applied across the anode and cathode, the used nuclear fuel is oxidized at the anode, thereby dissolving into the molten salt electrolyte. At the same time, at the cathode, electrons reduce solvated uranium and plate the uranium onto the cathode. The plated-out uranium can be removed and further purified for use as still-fissile material.
Concurrent with uranium plating onto the cathode, noble metal fission products remain in the anode basket, while most of the other fission products from the used uranic nuclear fuel remain in solution in the electrolyte. These fission products include alkali-, alkaline earth-, rare earth-, and halogen-containing compounds. Used uranic nuclear fuel also contains transuranic elements. All transuranic elements dissolve into the molten salt electrolyte and will co-deposit with uranium on another cathode specifically designed for their recovery. Frequently, after the uranic used nuclear fuel is processed in this way, the electrochemical salts that now contain waste fission products are processed, with the salt being recycled to the process and the waste fission products being incorporated into waste forms for storage in long-term radioactive material repositories.
Available methods for in situ monitoring of electrochemical processing of nuclear materials are limited. High temperatures (500-650° C.) of the molten salt electrolyte solution and radiation emanating from the actinides and fission products present corrosion and radiation barriers to using traditional analytical methods to analyze the bulk electrochemical salt solution in situ. Thus, in situ analysis of the electrochemical salts is generally limited to electroanalytical techniques such as voltammetry. However, molten salt electroanalytical techniques are still under development and so are of limited utility.
As a result of the radiation and high temperatures involved in electroprocessing of used nuclear fuel, elemental and isotopic analysis of electrochemical salts is generally done off-line. Currently, workers, or worker-controlled robots manually take samples of the bulk electrochemical salts from the electrorefiner using fritted tubes. The samples are then removed from the sampling implement, weighed, dissolved in water, and diluted for analysis by techniques such as mass spectrometry.
Manual sampling with off-line analysis has its drawbacks, including the significant time between sampling and analysis, high labor costs, and removing significantly more salt than necessary from an electrorefiner. Additionally, manual sampling of electrochemical salts only allows for a snapshot of the content of the electrochemical salts at the moment a sample is taken. There is also the possibility of a layer of dross on the surface of the electrolyte, which can interfere with the collection of a representative sample by the dip tube method. Taking infrequent manual samples that may not represent the content of the bulk of the electrochemical salts can introduce significant error to the results of monitoring an ongoing electrorefining process. Process monitoring errors can have negative consequences in both process control and in nuclear material accountancy.
A need exists in the art for an on-line, high-throughput method and system of automatically monitoring used nuclear fuel electrorefining processes. The method and system would automatically extract and provide samples of electrochemical process salts in nanoliter (nL, e.g. as little as 10 nL) to several milliliter (mL) volumes (e.g., as much as 10 ml) so as to eliminate exposure of analytical devices and workers to high-temperatures and high radiation levels.